Information About NAT (ASA 8.3 and Later)
This chapter provides an overview of how Network Address Translation (NAT) works on the adaptive security appliance. This chapter includes the following sections:
•Why Use NAT?
•NAT in Routed and Transparent Mode
•How NAT is Implemented
•NAT Rule Order
•Mapped Address Guidelines
•DNS and NAT
•Where to Go Next
Note To start configuring NAT, see Chapter 27 "Configuring Network Object NAT (ASA 8.3 and Later)," or Chapter 28 "Configuring Twice NAT (ASA 8.3 and Later)."
Why Use NAT?
Each computer and device within an IP network is assigned a unique IP address that identifies the host. Because of a shortage of public IPv4 addresses, most of these IP addresses are private, not routable anywhere outside of the private company network. RFC 1918 defines the private IP addresses you can use internally that should not be advertised:
•10.0.0.0 through 10.255.255.255
•172.16.0.0 through 172.31.255.255
•192.168.0.0 through 192.168.255.255
One of the main functions of NAT is to enable private IP networks to connect to the Internet. NAT replaces a private IP address with a public IP address, translating the private addresses in the internal private network into legal, routable addresses that can be used on the public Internet. In this way, NAT conserves public addresses because it can be configured to advertise only one public address for the entire network to the outside world.
Other functions of NAT include:
•Security—Keeping internal IP addresses hidden discourages direct attacks.
•IP routing solutions—Overlapping IP addresses are not a problem when you use NAT.
•Flexibility—You can change internal IP addressing schemes without affecting the public addresses available externally; for example, for a server accessible to the Internet, you can maintain a fixed IP address for Internet use, but internally, you can change the server address.
Note NAT is not required. If you do not configure NAT for a given set of traffic, that traffic will not be translated, but will have all of the security policies applied as normal.
This document uses the following terminology:
•Real address/host/network/interface—The real address is the address that is defined on the host, before it is translated. In a typical NAT scenario where you want to translate the inside network when it accesses the outside, then the inside network would be the "real" network. Note that you can translate any network connected to the adaptive security appliance, not just an inside network, Therefore if you configure NAT to translate outside addresses, "real" can refer to the outside network when it accesses the inside network.
•Mapped address/host/network/interface—The mapped address is the address that the real address is translated to. In a typical NAT scenario where you want to translate the inside network when it accesses the outside, then the outside network would be the "mapped" network.
•Bidirectional initiation—Static NAT allows connections to be initiated bidirectionally, meaning both to the host and from the host.
•Source and destination NAT—For any given packet, both the source and destination IP addresses are compared to the NAT rules, and one or both can be translated/untranslated.
You can implement NAT using the following methods:
•Static NAT—A consistent mapping between a real and mapped IP address. Allows bidirectional traffic initiation.
•Dynamic NAT—A group of real IP addresses are mapped to a (usually smaller) group of mapped IP addresses, on a first come, first served basis. Only the real host can initiate traffic.
•Dynamic Port Address Translation (PAT)—A group of real IP addresses are mapped to a single IP address using a unique source port of that IP address.
•Identity NAT—Static NAT lets you translate a real address to itself, essentially bypassing NAT. You might want to configure NAT this way when you want to translate a large group of addresses, but then want to exempt a smaller subset of addresses.
This section includes the following topics:
This section describes static NAT and includes the following topics:
•Information About Static NAT
•Information About Static NAT with Port Translation
•Information About One-to-Many Static NAT
•Information About Other Mapping Scenarios (Not Recommended)
Information About Static NAT
Static NAT creates a fixed translation of a real address to a mapped address. Because the mapped address is the same for each consecutive connection, static NAT allows bidirectional connection initiation, both to and from the host (if an access rule exists that allows it). With dynamic NAT and PAT, on the other hand, each host uses a different address or port for each subsequent translation, so bidirectional initiation is not supported.
Figure 26-1 shows a typical static NAT scenario. The translation is always active so both real and remote hosts can initiate connections.
Figure 26-1 Static NAT
Information About Static NAT with Port Translation
Static NAT with port translation lets you specify a real and mapped protocol (TCP or UDP) and port.
This section includes the following topics:
•Information About Static NAT with Port Address Translation
•Static NAT with Identity Port Translation
•Static NAT with Port Translation for Non-Standard Ports
•Static Interface NAT with Port Translation
Information About Static NAT with Port Address Translation
When you specify the port with static NAT, you can choose to map the port to the same value or to a different value. Using the same value lets you translate ipA/port1 to ipX/port1 while translating ipA/port2 to ipY/port2.
Figure 26-2 shows a typical static NAT with port translation scenario showing both a port that is mapped to itself and a port that is mapped to a different value. The translation is always active so both translated and remote hosts can initiate connections.
Figure 26-2 Typical Static NAT with Port Translation Scenario
Note For applications that require application inspection for secondary channels (for example, FTP and VoIP), the adaptive security appliance automatically translates the secondary ports.
Static NAT with Identity Port Translation
The following static NAT with port translation example provides a single address for remote users to access FTP, HTTP, and SMTP. These servers are actually different devices on the real network, but for each server, you can specify static NAT with port translation rules that use the same mapped IP address, but different ports. (See Figure 26-3. See the "Single Address for FTP, HTTP, and SMTP (Static NAT with Port Translation)" section for details on how to configure this example.)
Figure 26-3 Static NAT with Port Translation
Static NAT with Port Translation for Non-Standard Ports
You can also use static NAT with port translation to translate a well-known port to a non-standard port or vice versa. For example, if inside web servers use port 8080, you can allow outside users to connect to port 80, and then undo translation to the original port 8080. Similarly, to provide extra security, you can tell web users to connect to non-standard port 6785, and then undo translation to port 80.
Static Interface NAT with Port Translation
You can configure static NAT to map a real address to an interface address/port combination. For example, if you want to redirect Telnet access to the adaptive security appliance outside port to an inside IP address (for example, a router interface), then you can map the inside IP address on port 23 to the interface address on port 23. (Note that Telnet is not allowed to the lowest security interface normally; static NAT with interface port translation redirects the disallowed Telnet session instead of denying it).
Information About One-to-Many Static NAT
Typically, you configure static NAT with a one-to-one mapping. However, in some cases, you might want to configure a single real address to several mapped addresses (one-to-many). When you configure one-to-many static NAT, when the real host initiates traffic, it always uses the first mapped address. However, for traffic initiated to the host, you can initiate traffic to any of the mapped addresses, and they will be untranslated to the single real address.
Figure 26-4 shows a typical one-to-many static NAT scenario. The first translation is always active so both translated and remote hosts can initiate connections, but the subsequent mappings are unidirectional to the real host.
Figure 26-4 One-to-Many Static NAT
For example, you have a load balancer at 10.1.2.27; depending on the URL requested, it redirects traffic to the correct web server. (See Figure 26-5. See the "Inside Load Balancer with Multiple Mapped Addresses (Static NAT, One-to-Many)" section for details on how to configure this example.)
Figure 26-5 One-to-Many Static NAT
Information About Other Mapping Scenarios (Not Recommended)
The adaptive security appliance has the flexibility to allow any kind of static mapping scenario: one-to-one, one-to-many, but also few-to-many, many-to-few, and many-to-one mappings. These other mapping options, however, might result in unintended consequences. We recommend using only one-to-one or one-to-many mappings.
Functionally, few-to-many is the same as one-to-many; but because the configuration is more complicated, we recommend creating a one-to-many configuration for each real address that requires it. For example, for a few-to-many scenario, the few real addresses are mapped to the many mapped addresses in order (A to 1, B to 2, C to 3). When all real addresses are mapped, the next mapped address is mapped to the first real address, and so on until all mapped addresses are mapped (A to 4, B to 5, C to 6). This results in multiple mapped addresses for each real address. Just like a one-to-many configuration, only the first mappings are bidirectional; subsequent mappings allow traffic to be initiated to the real host, but all traffic from the real host uses only the first mapped address for the source.
Figure 26-6 shows a typical few-to-many static NAT scenario. The first translation for each real address is always active so both translated and remote hosts can initiate connections, but the subsequent mappings are unidirectional to the real hosts.
Figure 26-6 Few-to-Many Static NAT
For a many-to-few or many-to-one configuration, where you have more real addresses than mapped addresses, you run out of mapped addresses before you run out of real addresses. Only the mappings between the lowest real IP addresses and the mapped pool result in bidirectional initiation. The remaining higher real addresses can initiate traffic, but traffic cannot be initiated to them (returning traffic for a connection is directed to the correct real address because of the unique 5-tuple (source IP, destination IP, source port, destination port, protocol) for the connection).
Note Many-to-few or many-to-one NAT is not PAT. If two real hosts use the same source port number and go to the same outside server and the same TCP destination port, and both hosts are translated to the same IP address, then both connections will be reset because of an address conflict (the 5-tuple is not unique).
Figure 26-7 shows a typical many-to-few static NAT scenario. The translations between the lowest real addresses and the mapped addresses are always active so both translated and remote hosts can initiate connections, but the mappings for higher IP addresses are unidirectional from the real hosts.
Figure 26-7 Many-to-Few Static NAT
Instead of using a static rule this way, we suggest that you create a one-to-one rule for the traffic that needs bidirectional initiation, and then create a dynamic rule for the rest of your addresses.
This section describes dynamic NAT and includes the following topics:
•Information About Dynamic NAT
•Dynamic NAT Disadvantages and Advantages
Information About Dynamic NAT
Dynamic NAT translates a group of real addresses to a pool of mapped addresses that are routable on the destination network. The mapped pool typically includes fewer addresses than the real group. When a host you want to translate accesses the destination network, the adaptive security appliance assigns the host an IP address from the mapped pool. The translation is created only when the real host initiates the connection. The translation is in place only for the duration of the connection, and a given user does not keep the same IP address after the translation times out. Users on the destination network, therefore, cannot initiate a reliable connection to a host that uses dynamic NAT, even if the connection is allowed by an access rule.
Figure 26-8 shows a typical dynamic NAT scenario. Only real hosts can create a NAT session, and responding traffic is allowed back.
Figure 26-8 Dynamic NAT
Figure 26-9 shows a remote host attempting to initiate a connection to a mapped address. This address is not currently in the translation table; therefore, the adaptive security appliance drops the packet.
Figure 26-9 Remote Host Attempts to Initiate a Connection to a Mapped Address
Note For the duration of the translation, a remote host can initiate a connection to the translated host if an access rule allows it. Because the address is unpredictable, a connection to the host is unlikely. Nevertheless, in this case you can rely on the security of the access rule.
Dynamic NAT Disadvantages and Advantages
Dynamic NAT has these disadvantages:
•If the mapped pool has fewer addresses than the real group, you could run out of addresses if the amount of traffic is more than expected.
Use PAT if this event occurs often because PAT provides over 64,000 translations using ports of a single address.
•You have to use a large number of routable addresses in the mapped pool; if the destination network requires registered addresses, such as the Internet, you might encounter a shortage of usable addresses.
The advantage of dynamic NAT is that some protocols cannot use PAT. PAT does not work with the following:
•IP protocols that do not have a port to overload, such as GRE version 0.
•Some multimedia applications that have a data stream on one port, the control path on another port, and are not open standard.
See the "When to Use Application Protocol Inspection" section for more information about NAT and PAT support.
This section describes dynamic PAT and includes the following topics:
•Information About Dynamic PAT
•Dynamic PAT Disadvantages and Advantages
Information About Dynamic PAT
Dynamic PAT translates multiple real addresses to a single mapped IP address by translating the real address and source port to the mapped address and a unique port above 1024. Each connection requires a separate translation session because the source port differs for each connection. For example, 10.1.1.1:1025 requires a separate translation from 10.1.1.1:1026.
Figure 26-10 shows a typical dynamic PAT scenario. Only real hosts can create a NAT session, and responding traffic is allowed back. The mapped address is the same for each translation, but the port is dynamically assigned.
Figure 26-10 Dynamic PAT
After the connection expires, the port translation also expires after 30 seconds of inactivity. The timeout is not configurable. Users on the destination network cannot reliably initiate a connection to a host that uses PAT (even if the connection is allowed by an access rule).
Note For the duration of the translation, a remote host can initiate a connection to the translated host if an access rule allows it. Because the port address (both real and mapped) is unpredictable, a connection to the host is unlikely. Nevertheless, in this case you can rely on the security of the access rule.
Dynamic PAT Disadvantages and Advantages
Dynamic PAT lets you use a single mapped address, thus conserving routable addresses. You can even use the adaptive security appliance interface IP address as the PAT address.
Dynamic PAT does not work with some multimedia applications that have a data stream that is different from the control path. See the "When to Use Application Protocol Inspection" section for more information about NAT and PAT support.
You might have a NAT configuration in which you need to translate an IP address to itself. For example, if you create a broad rule that applies NAT to every network, but want to exclude one network from NAT, you can create a static NAT rule to translate an address to itself. Identity NAT is necessary for remote access VPN, where you need to exempt the client traffic from NAT.
Note Identity NAT does not perform proxy ARP nor does it allow the specified interface to override the route lookup for a packet.
Figure 26-11 shows a typical identity NAT scenario.
Figure 26-11 Identity NAT
NAT in Routed and Transparent Mode
You can configure NAT in both routed and transparent firewall mode. This section describes typical usage for each firewall mode and includes the following topics:
•NAT in Routed Mode
•NAT in Transparent Mode
NAT in Routed Mode
Figure 26-12 shows a typical NAT example in routed mode, with a private network on the inside.
Figure 26-12 NAT Example: Routed Mode
1. When the inside host at 10.1.2.27 sends a packet to a web server, the real source address of the packet, 10.1.2.27, is changed to a mapped address, 184.108.40.206.
2. When the server responds, it sends the response to the mapped address, 220.127.116.11, and the adaptive security appliance receives the packet.
3. The adaptive security appliance then changes the translation of the mapped address, 18.104.22.168, back to the real address, 10.1.2.27, before sending it to the host.
NAT in Transparent Mode
Using NAT in transparent mode eliminates the need for the upstream or downstream routers to perform NAT for their networks.
NAT in transparent mode has the following requirements and limitations:
•When the mapped addresses are not on the same network as the transparent firewall, then on the upstream router you need to add a static route for the mapped addresses that points to the downstream router (through the adaptive security appliance).
•When you have VoIP or DNS traffic with NAT and inspection enabled, to successfully translate the IP address inside VoIP and DNS packets, the adaptive security appliance needs to perform a route lookup. Unless the host is on a directly-connected network, then you need to add a static route on the adaptive security appliance for the real host address that is embedded in the packet.
•Because the transparent firewall does not have any interface IP addresses, you cannot use interface PAT.
•ARP inspection is not supported. Moreover, if for some reason a host on one side of the adaptive security appliance sends an ARP request to a host on the other side of the adaptive security appliance, and the initiating host real address is mapped to a different address on the same subnet, then the real address remains visible in the ARP request.
Figure 26-13 shows a typical NAT scenario in transparent mode, with the same network on the inside and outside interfaces. The transparent firewall in this scenario is performing the NAT service so that the upstream router does not have to perform NAT.
Figure 26-13 NAT Example: Transparent Mode
1. When the inside host at 10.1.1.75 sends a packet to a web server, the real source address of the packet, 10.1.1.75, is changed to a mapped address, 22.214.171.124.
2. When the server responds, it sends the response to the mapped address, 126.96.36.199, and the adaptive security appliance receives the packet because the upstream router includes this mapped network in a static route directed through the adaptive security appliance.
3. The adaptive security appliance then undoes the translation of the mapped address, 188.8.131.52, back to the real address, 10.1.1.1.75. Because the real address is directly-connected, the adaptive security appliance sends it directly to the host.
4. For host 192.168.1.2, the same process occurs, except that the adaptive security appliance looks up the route in its route table and sends the packet to the downstream router at 10.1.1.3 based on the static route.
How NAT is Implemented
The adaptive security appliance can implement address translation in two ways: network object NAT and twice NAT. This section includes the following topics:
•Main Differences Between Network Object NAT and Twice NAT
•Information About Network Object NAT
•Information About Twice NAT
Main Differences Between Network Object NAT and Twice NAT
The main differences between these two NAT types are:
•How you define the real address.
–Network object NAT—You define NAT as a parameter for a network object; the network object definition itself provides the real address. This method lets you easily add NAT to network objects. The objects can also be used in other parts of your configuration, for example, for access rules or even in twice NAT rules.
–Twice NAT—You identify a network object or network object group for both the real and mapped addresses. In this case, NAT is not a parameter of the network object; the network object or group is a parameter of the NAT configuration. The ability to use a network object group for the real address means that twice NAT is more scalable.
•How source and destination NAT is implemented.
–Network object NAT— Each rule can apply to either the source or destination of a packet. So two rules might be used, one for the source IP address, and one for the destination IP address. These two rules cannot be tied together to enforce a specific translation for a source/destination combination.
–Twice NAT—A single rule translates both the source and destination. A matching packet only matches the one rule, and further rules are not checked. Even if you do not configure the optional destination address for twice NAT, a matching packet still only matches one twice NAT rule. The source and destination are tied together, so you can enforce different translations depending on the source/destination combination. For example, sourceA/destinationA can have a different translation than sourceA/destinationB.
•Order of NAT Rules.
–Network object NAT—Automatically ordered in the NAT table.
–Twice NAT—Manually ordered in the NAT table (before or after network object NAT rules).
See the "NAT Rule Order" section for more information.
We recommend using network object NAT unless you need the extra features that twice NAT provides. Network object NAT is easier to configure, and might be more reliable for applications such as Voice over IP (VoIP). (For VoIP, because twice NAT is applicable only between two objects, you might see a failure in the translation of indirect addresses that do not belong to either of the objects.)
Information About Network Object NAT
All NAT rules that are configured as a parameter of a network object are considered to be network object NAT rules. Network object NAT is a quick and easy way to configure NAT for a network object, which can be a single IP address, a range of addresses, or a subnet.
After you configure the network object, you can then identify the mapped address for that object, either as an inline address or as another network object or network object group.
When a packet enters the adaptive security appliance, both the source and destination IP addresses are checked against the network object NAT rules. The source and destination address in the packet can be translated by separate rules if separate matches are made. These rules are not tied to each other; different combinations of rules can be used depending on the traffic.
Because the rules are never paired, you cannot specify that sourceA/destinationA should have a different translation than sourceA/destinationB. Use twice NAT for that kind of functionality (twice NAT lets you identify the source and destination address in a single rule).
To start configuring network object NAT, see Chapter 27 "Configuring Network Object NAT (ASA 8.3 and Later)."
Information About Twice NAT
Twice NAT lets you identify both the source and destination address in a single rule. Specifying both the source and destination addresses lets you specify that sourceA/destinationA can have a different translation than sourceA/destinationB.
The destination address is optional. If you specify the destination address, you can either map it to itself (identity NAT), or you can map it to a different address. The destination mapping is always a static mapping.
Twice NAT also lets you use service objects for static NAT with port translation; network object NAT only accepts inline definition.
To start configuring twice NAT, see Chapter 28 "Configuring Twice NAT (ASA 8.3 and Later)."
Figure 26-14 shows a host on the 10.1.2.0/24 network accessing two different servers. When the host accesses the server at 184.108.40.206, the real address is translated to 220.127.116.11. When the host accesses the server at 18.104.22.168, the real address is translated to 22.214.171.124. (See the "Single Address for FTP, HTTP, and SMTP (Static NAT with Port Translation)" section for details on how to configure this example.)
Figure 26-14 Twice NAT with Different Destination Addresses
Figure 26-15 shows the use of source and destination ports. The host on the 10.1.2.0/24 network accesses a single host for both web services and Telnet services. When the host accesses the server for web services, the real address is translated to 126.96.36.199. When the host accesses the same server for Telnet services, the real address is translated to 188.8.131.52.
Figure 26-15 Twice NAT with Different Destination Ports
Figure 26-16 shows a remote host connecting to a translated host. The translated host has a twice static NAT translation that translates the real address only for traffic to and from the 184.108.40.206/27 network. A translation does not exist for the 220.127.116.11/27 network, so the translated host cannot connect to that network, nor can a host on that network connect to the translated host.
Figure 26-16 Twice Static NAT with Destination Address Translation
NAT Rule Order
Network object NAT rules and twice NAT rules are stored in a single table that is divided into three sections. Section 1 rules are applied first, then section 2, and finally section 3. Table 26-1 shows the order of rules within each section.
Table 26-1 NAT Rule Table
Order of Rules within the Section
Applied on a first match basis, in the order they appear in the configuration. By default, twice NAT rules are added to section 1.
Note If you configure VPN, the client dynamically adds invisible NAT rules to the end of this section. Be sure that you do not configure a twice NAT rule in this section that might match your VPN traffic, instead of matching the invisible rule. If VPN does not work due to NAT failure, consider adding twice NAT rules to section 3 instead.
Network object NAT
Section 2 rules are applied in the following order, as automatically determined by the adaptive security appliance:
1. Static rules.
2. Dynamic rules.
Within each rule type, the following ordering guidelines are used:
a. Quantity of real IP addresses—From smallest to largest. For example, an object with one address will be assessed before an object with 10 addresses.
b. For quantities that are the same, then the IP address number is used, from lowest to highest. For example, 10.1.1.0 is assessed before 18.104.22.168.
c. If the same IP address is used, then the name of the network object is used, in alphabetical order. For example, abracadabra is assessed before catwoman.
Section 3 rules are applied on a first match basis, in the order they appear in the configuration. You can specify whether to add a twice NAT rule to section 3 when you add the rule.
For section 2 rules for example, you have the following IP addresses defined within network objects:
172.16.1.0/24 (dynamic) (object def)
172.16.1.0/24 (dynamic) (object abc)
The resultant ordering would be:
172.16.1.0/24 (dynamic) (object abc)
172.16.1.0/24 (dynamic) (object def)
You can configure a NAT rule to apply to any interface, or you can identify specific real and mapped interfaces. You can also specify any interface for the real address, and a specific interface for the mapped address, or vice versa.
For example, you might want to specify any interface for the real address and specify the outside interface for the mapped address if you use the same private addresses on multiple interfaces, and you want to translate them all to the same global pool when accessing the outside (Figure 26-17).
Figure 26-17 Specifying Any Interface
Mapped Address Guidelines
When you translate the real address to a mapped address, you can use the following mapped addresses:
•Addresses on the same network as the mapped interface.
If you use addresses on the same network as the mapped interface (through which traffic exits the adaptive security appliance), the adaptive security appliance uses proxy ARP to answer any requests for mapped addresses, and thus it intercepts traffic destined for a real address. This solution simplifies routing because the adaptive security appliance does not have to be the gateway for any additional networks. However, this approach does put a limit on the number of available addresses used for translations.
For PAT, you can even use the IP address of the mapped interface.
Note If you configure the mapped interface to be any interface, but you specify a mapped address on the same network as one of the interfaces, then if an ARP request for that mapped address comes in on a different interface, then you need to manually configure an ARP entry for that network on the other interface where you specify the interface MAC address (see Configuration > Device Management > Advanced > ARP > ARP Static Table). Typically, if you specify any interface for the mapped interface, then you use a unique network for the mapped addresses.
•Addresses on a unique network.
If you need more addresses than are available on the mapped interface network, you can identify addresses on a different subnet. The adaptive security appliance uses proxy ARP to answer any requests for mapped addresses, and thus it intercepts traffic destined for a real address.
Note Identity NAT does not perform proxy ARP nor does it allow the specified interface to override the route lookup for a packet.
See additional guidelines about mapped IP addresses in Chapter 27 "Configuring Network Object NAT (ASA 8.3 and Later)," and Chapter 28 "Configuring Twice NAT (ASA 8.3 and Later)."
DNS and NAT
You might need to configure the adaptive security appliance to modify DNS replies by replacing the address in the reply with an address that matches the NAT configuration. You can configure DNS modification when you configure each translation.
This feature rewrites the A record, or address record, in DNS replies that match a NAT rule. For DNS replies traversing from a mapped interface to any other interface, the A record is rewritten from the mapped value to the real value. Inversely, for DNS replies traversing from any interface to a mapped interface, the A record is rewritten from the real value to the mapped value.
Note If you configure a twice NAT rule, you cannot configure DNS modification if you specify the source address as well as the destination address. These kinds of rules can potentially have a different translation for a single address when going to A vs. B. Therefore, the adaptive security appliance cannot accurately match the IP address inside the DNS reply to the correct twice NAT rule; the DNS reply does not contain information about which source/destination address combination was in the packet that prompted the DNS request.
For example, a DNS server is accessible from the outside interface. A server, ftp.cisco.com, is on the inside interface. You configure the adaptive security appliance to statically translate the ftp.cisco.com real address (10.1.3.14) to a mapped address (22.214.171.124) that is visible on the outside network. (See Figure 26-18.) In this case, you want to enable DNS reply modification on this static rule so that inside users who have access to ftp.cisco.com using the real address receive the real address from the DNS server, and not the mapped address.
When an inside host sends a DNS request for the address of ftp.cisco.com, the DNS server replies with the mapped address (126.96.36.199). The adaptive security appliance refers to the static rule for the inside server and translates the address inside the DNS reply to 10.1.3.14. If you do not enable DNS reply modification, then the inside host attempts to send traffic to 188.8.131.52 instead of accessing ftp.cisco.com directly.
Figure 26-18 DNS Reply Modification
Note If a user on a different network (for example, DMZ) also requests the IP address for ftp.cisco.com from the outside DNS server, then the IP address in the DNS reply is also modified for this user, even though the user is not on the Inside interface referenced by the static rule.
Figure 26-19 shows a web server and DNS server on the outside. The adaptive security appliance has a static translation for the outside server. In this case, when an inside user requests the address for ftp.cisco.com from the DNS server, the DNS server responds with the real address, 184.108.40.206. Because you want inside users to use the mapped address for ftp.cisco.com (10.1.2.56) you need to configure DNS reply modification for the static translation.
Figure 26-19 DNS Reply Modification Using Outside NAT
Where to Go Next
To configure network object NAT, see Chapter 27 "Configuring Network Object NAT (ASA 8.3 and Later)."
To configure twice NAT, see Chapter 28 "Configuring Twice NAT (ASA 8.3 and Later)."